In a typical compressor for a cryogenic refrigerator, helium returns from a cryogenic refrigerator to a compressor pump via a helium return line. Oil is injected into the helium at the inlet to the compressor. The oil absorbs the heat of compression given off by the helium. The combined mixture of helium and oil is pumped from the compressor through a line to a heat exchanger where the heat contained in the mixture is given off. The helium and oil mixture is then pumped to a bulk oil separator which separates the helium from the oil and the oil returns via a line back to the compressor. The helium travels from the separator to an oil mist separator where any residual oil mist is separated from the helium.
The helium travels from the oil mist separator to an adsorber which further removes any remaining impurities from the helium. From the adsorber, the helium is then pumped via a helium supply line to the cold head of a cryogenic refrigerator such as a Gifford-McMahon cryogenic refrigerator, where it expands to a lower pressure. The lower pressure helium travels returns via the helium return line back to the compressor where the cycle is again repeated.
An additional helium line lies between the helium supply line and the helium return line. Situated within this line is a differential-pressure relief or by-pass valve. Any excess pressure which may build up in the helium supply line to the cryogenic refrigerator can be released through this line and valve and shunted to the helium return line valve. The relief valve automatically opens and allows helium to travel from the supply line to the return line when the pressure difference between the helium supply line and the helium return line reaches a given predetermined pre-set pressure. The setting on the by-pass valve is determined by the maximum pressure difference at which the compressor pump can operate under worst case conditions (for example, voltage, ambient, water temperature, and flow rate).
With the use of larger cryogenic refrigerator systems used in the field of manufacturing of semiconductors, for example, it is desirable to match the system demand, which often varies depending on the load, with the compressor output to optimize efficiency of the system. It has been shown that raising the operating pressure in the system can increase efficiency of a Gifford-McMahon refrigeration system. When a system was charged at the high pressure level, the setting on the bypass valve had to be reduced from 235 psi to 210 psi to prevent overheating the compressor motor.
FIG. 1 depicts the compressor operating at low pressure. The y axis on the left hand side of the graph measures the flow rate of gas through the compressor illustrated by line 2. The x axis measures the pressure differential between the low and high side pressures in the system. The y axis on the right hand side measures power in watts that the compressor consumes illustrated by line 4. The bypass valve in this embodiment is not fully closed until about 200 psi differential or below. FIG. 2 is similar to FIG. 1 but depicts the compressor operating at high pressure in which the flow rate is substantially greater than at low pressure. Because the bypass valve is set at about 210 psi, the valve does not close fully until the pressure differential falls below about 180 psi.
In accordance with the present invention, a method is provided for controlling a system pressure in a refrigeration system based on a variable load, which includes sensing return pressure and high side pressure in the system, and adjusting the return pressure to optimize a gas flow rate in the system by adding or removing gas from the system through an operating range of pressures in response to the sensed return pressure and the sensed high side pressure. The method can further include calculating a pressure difference between the return pressure and the high side pressure.
A second pressure difference can be calculated, and if the pressure difference decreases, gas is added to system and if the pressure difference increases, gas is removed from the system.
In alternative embodiments, the pressure difference between the return pressure and the high side pressure can be sensed, for example, with a differential pressure gauge. The low pressure can be adjusted to optimize the gas flow rate in the system by adding or removing gas from the system through the operating range of pressures in response to the sensed return or high side pressure and the sensed pressure difference.
An apparatus is also provided for optimizing a gas flow rate in a refrigeration system, comprising a compressor pump for compressing a gas, and at least one cold head that receives the compressed gas from a supply line and allows the gas to expand and to be returned to the compressor pump by a return line. The apparatus further includes a gas volume disposed between the supply line and return line for adding or removing gas from the system to optimize the flow rate of the gas in the system through an operating range of pressures in response to sensed pressures in the supply line and return line. In one embodiment, the refrigeration system is a cryogenic refrigeration system and the gas includes helium.
A first valve can be disposed on a high pressure side of the gas volume and a second valve can be disposed on a low pressure side of the gas volume for controlling the flow rate of the gas in the system. A first sensing device can be disposed on the high pressure side of the gas volume for sensing a supply pressure and a second sensing device can be disposed on the low pressure side of the gas volume for sensing a return pressure. A controller can be coupled to the sensing devices for receiving the supply pressure and return pressure and calculating the pressure difference.
A first actuator can be coupled to the first valve for opening and closing the same and a second actuator can be coupled to the second valve for opening and closing the same in response to commands from the controller. If the pressure difference decreases, the controller directs the first actuator to close the first valve and directs the second actuator to open the second valve to allow gas to enter the system. If the pressure difference increases, the controller directs the first actuator to open the first valve and directs the second actuator to close the second valve to allow gas to be removed from the system.
The benefits of the present invention are illustrated in FIG. 3. As system demand decreases, that is, less flow is demanded of the compressor by the cryopumps, power is reduced dramatically, for example, from about 6000 watts to about 4600 watts at a 0 SCFM (standard cubic feet per minute) flow rate. Further, the compressor is capable of providing full output, i.e., the bypass valve is fully closed, at a high pressure differential (200 psi vs. 180 psi). Generally, the invention allows the system to operate at both high and low pressures and every pressure in between, and controls this operation based on demand of the cryopumps.